Hopping it to Proxima

By Martin Lewicki (Adelaide Planetarium)

One of the most celebrated stars of modern visual astronomy is the binary star system Alpha Centauri. Both serious and novice observers often turn their telescopes toward this admirable southern showpiece. Replete with a list of Alpha’s special attributes we revel over its close 4.4 light years distance from Earth, the 79.9-year orbital dance of its two components, their similarity to the size and brightness of our Sun, and how strange a sky might look to any inhabitants of a world in orbit within this system with two suns in the sky. At magnitude –0.01 Alpha Centauri is easy to find as the brightest of the two “Pointers” pointing at the Southern Cross and makes a good beginner’s launch pad for learning constellations. One wonders how the Robinson Family of the 60’s “Lost in Space” TV series could have possibly got lost on the way to such a bright and obvious nearby target!
(Author Martin Lewicki at UniSA)

Alpha Centauri’s celebrity status however overshadows that of its faint red dwarf “companion” Proxima Centauri only 1.8 degrees to the south of Alpha. At magnitude 11.1 Proxima is buried in a field of stars with a similar magnitude range making it difficult to identify, even with a Goto telescope. While a Goto will find Proxima’s star field, it may not be exactly centred in the eyepiece and still requires a knowledge of the field for it to be identified. Until Gotos evolve with flashing cursors in the eyepiece that say “here it is”, to most observers a detailed sky map of the field and a star-hop is still the way to reach this diminutive reddish stellar ember.

Proxima Centauri is a type M red dwarf weighing in 12% of the Sun’s mass and 15% of its diameter. It has a surface temperature radiating at a moderately “cool” 3040° Celsius, about the temperature of the tungsten filament in a 60-watt light globe. Proxima is a little closer than Alpha at 4.2 lights year making it the closest star to our solar system. It is 0.186 light years from Alpha Centauri itself. Proxima was born in the same nebula as Alpha Centauri but, contrary to popular belief, is not in a closed orbit around Alpha. Its relative space motion is seven times Alpha’s escape velocity and so is part of a disintegrating cluster that includes Alpha and other nearby stars. Our Sun, by the way, was born more recently (4.6 billion years old) in a separate nebula and is not related the Alpha Centauri system (6.5 billion years old). Proxima will however outlive both Alpha stars (and the Sun) by some 200 times their life spans — a consequence of a red dwarf’s low mass that results in a long-term, conservative nuclear burning cycle.
(Image of Proxima Centauri © David Malin, taken with the 1.2 metre UKS, AAO)

Proxima is also notable for its flare activity which has been monitored by amateur observers as well as astronomers in professional facilities such as the Parkes Radio Telescope, X-ray satellites, the Anglo Australian Telescope and Australia Telescope array. Most of the flare emission is in the X-ray part of the spectrum. But Proxima has also been observed to brighten by two visual magnitudes lasting only a few minutes at a time during a flare event. It would seem that only the most assiduous or very lucky observer would chance upon a sighting Proxima in flare!

During the last decade exo-planet hunters using the Faint Object Spectrograph on the Hubble Space Telescope have detected a possible companion in orbit around Proxima. Other measurements with the Fine Guidance Sensor on the Hubble and other studies have constrained the putative companion to less than one Jupiter mass at distance of 0.5 AU from Proxima in a 1-year orbital period and 7 magnitudes fainter than Proxima. Meanwhile the European Southern Observatory (ESO) finds no radial velocity signal for Proxima – normally a giveaway that a star is wobbling as it plays hammer-throw with an unseen companion.

From the position of Proxima our Sun would appear as yellowish-white magnitude 0.4 star in eastern Cassiopeia while the Alpha pair would dominate Proxima’s night sky “only” 11,772 astronomical units (AU) away shining at magnitudes -6.8 and -5.5. You would need a telescope to resolve their disks but if you stayed around a little while you would observe their separation change from 10 to 3 arc minutes during their eccentric 79.9 years orbital cycle. Furthermore you could track their space motion of 0.4 arc minutes per year as they drift from between Zeta Ophiuchi and Beta Scorpii toward Sagittarius. An inhabitant of Alpha Centauri looking back at Proxima would see a magnitude 4.3 reddish star crawling through Taurus perhaps disturbing Alpha’s equivalent of the Oort Cloud sending torrents of comets toward the Alpha Centauri system.

At this time of the year Centaurus is culminating at mid evening and presents the best opportunity to hunt down Proxima Centauri. At magnitude 11.2 it is challenging for my Meade ETX90 (9cm Maksutov). With this telescope I have picked up Proxima at 48x with averted vision from my suburban backyard — just 10 km from the city centre. Bumping up the magnification to 101x darkens the sky background in the eyepiece allowing Proxima to glimmer more obviously. In my 6-inch Newtonian Proxima is more easily detected with almost direct vision at 60x. Higher magnifications are not only a handy way to increase contrast between background sky and faint stars to make them more readily visible but to also bring out detail in deep sky objects. This is especially true for light polluted skies.

To help you find this elusive nearby of all stars we have two maps to accommodate the two common telescope optical viewing configurations
(click on picture to enlarge). If you are using a telescope or a finder with an odd number of reflecting surfaces like a Schmidt Cassegrain or a refractor with a diagonal, your view in the eyepiece will be laterally inverted (as in a mirror) often requiring tedious mental gymnastics to match the view with a regular uninverted star map. So our “flipped map” accompanying the uninverted map is provided to accommodate “image flipped” telescope users. However nowadays most computer planetarium software will happily print flipped and correctly oriented star maps to a scale of choice for use at the eyepiece. You may wish to use one of these instead. While veteran observers may have their own favourite star-hop routine to Proxima my maps should hopefully get you there without too much difficulty. The trick is to recognize asterisms along the way and even give them cute names like “The Kite, and “The Y”. Star hopping across faint fields requires some practice but is a skill worth developing. I find that I remember many asterisms along the way and later, I can recall them to quickly revisit the sites previous targets like Proxima faster than a “speeding” Goto!




Last updated 17th of March 2011